The present invention relates to a polyolefin resin composition containing inorganic fillers with no problems of unpleasant odor.
It is well known that polyolefin resins are liable to discoloration and generation of unpleasant odors when they are subjected to oxidative degradation. This phenomenon of oxidative degradation can be prevented, in the case of polyethylene resins, for example, by admixing 2,6-di-tert-butyl-p-cresol (abbreviated as BHT hereinafter) into the resin in an amount of 0.001 to 0.1% by weight. Even when articles of a polyethylene resin are fabricated or put to use under extreme temperature conditions, the addition of low-volatile, high-molecular weight phenolic antioxidants such as [methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane and the like is sufficiently effective for preventing the oxidative degradation of the resin. In the case of polypropylene resins, it is a generally accepted practice that the amount of BHT is increased to 0.1 to 1.0% by weight and it is combined with dilaurylthiodipropionate due to the increased tendency toward oxidation at the higher temperatures necessitated for fabrication of the resins.
In recent times, various kinds of inorganic fillers have been incorporated into the polyolefin resins to give a resin composition containing 5 to 80% by weight of the filler with modified or improved properties. One of the major problem in the shaped articles of these filler-incorporated polyolefin resin compositions is their increased tendency toward discoloration and generation of unpleasant odors by the oxidative degradation in comparison with filler-free polyolefin resin compositions. When such a resin composition is exposed to a high temperature atmosphere, in particular, the unpleasant odor is so strong that working environments are heavily polluted during fabrication of the resin compositions by processes such as extrusion molding, injection molding, vacuum forming and the like. They are also defective for the fabrication of table and kitchenware which comes into contact with hot foods or drinks, as well as for containers and wrappings for foods or drinks where the retention of the flavor is of utmost importance.
In the prior art, attempts were made to solve the above described problems in three different ways. The first of them is the careful selection of the filler incorporated into the resin composition as is exemplified by the use of talc which gives rise to almost no problem of unpleasant odors. This approach is effective in preventing the unpleasant odor but such effectiveness is achieved only with talc of very specific grade with limitations in the origin, chemical composition and shape of the particles. The procedure is not applicable to inorganic fillers of other types.
The second of the three ways is the pretreatment of the inorganic filler with a base to achieve and acid value (Lewis acid) higher than 3.3. The thus treated filler is incorporated into the selected polyolefin resin composition containing a phenolic antioxidant with the result that the unpleasant odor is effectively prevented. Although this method is effective in preventing unpleasant odors, it is not attractive industrially because of the necessity for the surface treatment of the inorganic filler either in a wet process or a dry process prior to the blending step increases the production cost.
The third of the three ways depends largely on the use of additives such as an antioxidant and the like as a deodorant. This approach is presently under active investigations because of the simplicity of the process and the versatility for a wide variety of inorganic fillers. Typical examples of the formulations employed in this method are the addition of 0.01 to 5 parts by weight of distearylpentaerithritol diphosphite into 100 parts by weight of a blend of a polyolefin resin and an inorganic filler and a further admixing of 0.01 to 5 parts by weight of tetrakis[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]meth ane or 1,3,5-tris(3'5'-di-tert-butyl-4'-hydroxybenzyl)-2,4,6-trimethylbenzene to the above blend with distearylpentaerithritol diphosphite. The effectiveness of this method for preventing unpleasant odors is limited, however, so that it is unsatisfactory from a practical viewpoint. Other methods following this third approach are all unsatisfactory for preventing unpleasant odors. These include the addition of higher fatty acids or metal salts thereof, glycerin esters of higher fatty acids and the like.